The invention relates to high pressure packings and more particularly to a high pressure packing for a valve stem.
Packing assemblies for sealing an annulus between the cylindrical surfaces of relatively movable members, such as the valve stem of a gate valve, conventionally employ layers of packing rings which when subjected to axial compression are urged into sealing engagement with the cylindrical surfaces. In many instances, when the packing assembly is subjected to preloading for tightening the sealing elements to better resist leakage, the individual packing rings are damaged by cocking or rolling. In a packing assembly such as disclosed in U.S. Pat. No. 4,177,998 a plastic sealing material is injected into the stuffing box between upper and lower sets of packing rings for effecting a radial expansion of the packing rings. With a typical pressure fitting, the plastic is oftentimes injected at a pressure which exceeds the strength of the packing rings thereby causing the packing rings to extrude from the packing chamber or stuffing box. In some instances, the plastic will leak past the uppermost packing ring of the lower set of packing rings thereby causing their collapse.
Another serious problem often encountered with packing assemblies which utilize metal expander rings as in the aforesaid patent derives from the tendency of the valve stem to move off center when side loads are applied as might occur from injection of the plastic sealing material or from off center loads in the valve chamber. The metal rings can therefore cause galling of the stem when there is stem movement such that the packing assembly is unable to effect a seal against the stem surface or the valve stem becomes locked and renders the valve inoperative.
It is, accordingly, an object of the invention to provide an improved packing assembly for valve stems, or the like, which comprises packing rings and back up members for effecting a seal and is resistant to the application of side loads on the stem as would lead to off centering and galling of the stem.
It is another object to provide an improved packing assembly for a valve stem, or the like, wherein the sealing elements can be initially tightened to resist leakage without causing distortion or collapse of the sealing elements.
It is a further object to provide an improved packing gland assembly for a valve stem, or the like, of the type which comprises packing rings and back up adapter members as sealing elements in a stuffing box and which more effectively prevents the extrusion of the packing rings from the stuffing box during initial pressurization of the assembly for effecting a seal and during the application of external pressures on the packing gland.
A still further object is to provide an improved packing assembly for a valve stem, or the like, which is of the type comprising packing rings as sealing elements in a packing chamber and wherein a semi-fluid plastic is injectible into the packing chamber for effecting an initial pressurization of the packing assembly with a radial expansion and tightening of the sealing elements to resist leakage without causing the distortion or collapse of the packing rings during the initial pressurization.